The present invention relates to a protective coating for steel parts used in the manufacture of cranes. It also relates to a method of grafting a protective coating onto metallic parts that not only protects the part from corrosion and other adverse effects of the environmental conditions of temperature, pressure, humidity, corrosive gases like hydrogen sulfide, carbon dioxide, sulfur dioxide but also imparts an excellent degree of abrasion resistance.
The current process for the production of steel parts used in the manufacture of cranes doesn""t apply organic coatings on the finished products so as to protect them against corrosion, abrasion, impact, under environmental conditions of temperature, pressures, humidity, salt solutions of low and high pH and high concentration of corrosive gases like hydrogen sulfide, carbon dioxide, sulfur oxide and other gases. This is particularly true when the steel cranes are continuously used outdoors in the open air at ambient conditions of temperature and pressure, where they are being subjected to wear and tear and more importantly undergo corrosion. This may be due to the fact that water, water vapor or moisture readily dissolves a small amount of oxygen from the air in the solution and this, when condensed on the surface of the steel substrate, contributes to corrosion. For such a situation, the present instant invention relates to a process and composition for graft polymerizing a protective coating onto metallic substrate, so that the protection is sufficiently durable and is not readily removed from the metal as it undergoes abrasion and exposure to the environmental conditions during routine use.
Presently many different types of surface coatings are used in an effort to achieve the necessary requirements in terms of protection against corrosion and other properties. These include protective paints, yellow and clear chromatic coatings, and precious and non precious metal platings. Each of these methods have coat or performance deficiencies. Lack of surface resistivity to wear, abrasion, corrosion, impact, adhesion and durability resistance are all problems associated with these organic paints and coatings. The metal platings are generally expensive, and in some cases, are not tenacious enough for the abrasive environments of corrosion liquids and gases encountered by these steel products. As these coatings are applied at a minimal thickness and are physically rather than chemically bonded to the metallic substrate, they are susceptible to removal by normal abrasion when used in working conditions in outdoor environments. These platings are also susceptible to galvanic corrosion when contacting dissimilar metal surfaces. There is also a phenomena known as xe2x80x9cfretting corrosionxe2x80x9d which occurs with minimal mating pressures and is associated with metal migration/loss across the boundary area. In these cases, base metal corrosion occurs and environmental protection is lost. Also, in some instances, aesthetics are impacted due to surface contamination and discoloration. Therefore, the tenacity of these coatings is not sufficient for the requirements as warranted by the present invention. Even when the metallic parts are coated with expensive organic coatings, the coatings tend to be permeable to various corrosive gases and liquids so that the requisite degree of corrosion protection is not obtained. Because these coatings adhere to the substrate only through physical bonds, they can be readily dislodged from the metallic substrate over a short period of time as moisture, oxygen, chlorine, hydrogen sulfide and other corrosive gases permeate beneath the coated polymeric film. In view of this, there is a need for a coating that not only protects the metal from corrosion but also is abrasion resistant and is tenacious with a high degree of durability.
U.S. Pat. No. 5,015,507 to Des Lauriero describes a method of converting a rusted surface to a durable one by using a reducing agent which will function to reduce trivalent iron to divalent iron; an organic monomer which is capable of polymerization in the presence of the reducing agent on the surface; and a free radical initiator, which will polymerize the organic monomer to form the durable coating.
U.S. Pat. No. 4,453,988 to Slater discloses a method for coating a rusted metallic surface that uses a meth(acrylic) monomer; a peroxide curing system; and drying oil. Both these coating systems suffer from poor adhesion to the metal surface, since no method is described for directly bonding the film to the metal.
A number of inventions have been described previous to this instant invention which address the bonding problem through grafting of the film. For example, in U.S. Pat. No. 4,105,811 by Horowitz, hereby incorporated by reference, grafting technology is described to achieve a transparent, impervious protective coating on aluminum. In this case a peroxide and silver ion are present to initiate and catalyze graft polymerization of monomers and prepolymers which attach to the aluminum surface.
U.S. Pat. No. 5,043,226 to Wiedeman, hereby incorporated by reference, described a method of grafting a conductive, tenacious and protective coating on a metallic substrate such as steel, aluminum iron, platinum or silver. This method employed one or more polymerizable monomers having at least two functionalities, one or more graft initiators containing nickel ion and a reducing agent capable of reducing nickel ion to nickel metal.
U.S. Pat. No. 4,106,955 to Horowitz, hereby incorporated by reference, relates to the coating of steel articles wherein monomers containing hydroxyl, carbonyl, amine, glycidyl or azirdinyl groups react with steel surface to produce a corrosion and abrasion resistant surface.
The present invention employs a coating composition which differs from those used previously and provides superior corrosion and abrasion resistance to those previous compositions now cited. The instant invention is particularly advantageous for steel cranes which because of specific metallurgical considerations require a polymerizable/graftable species and initiator combination which is adapted to reaction with that specific surface. It is felt that this process, using the composition specified, will have general utility in a number of applications. In addition, the superior bonding achieved will confer improved corrosion and abrasion resistance in other applications as well.
Chemical grafting involves the activation of the substrate. Once the substrate has been activated, chains of monomers linked by carbon-carbon bonds grow on the substrate as whiskers. These whiskers impart new and desirable properties permanently imparted to the substrate without damaging any of the existing positive characteristics of these materials. Many materials, both naturally occurring and synthetic, posses hydrogens which are more reactive than the xe2x80x9cbulk hydrogensxe2x80x9d, for example, the tertiary hydrogen in polypropylene (1) the amide hydrogen in proteins (2) and the hydroxyl hydrogen in polysaccharides (3). 
Graft initiators (G.I) have the capacity of removing these active hydrogens and concomitantly initiating the growth of polymer chains at the side from where the active hydrogen was removed.
In the case of polypropylene, this can be represented as follows: 
Where * can represent a free radical, anion or cation, depending on whether the G.I. removes a hydrogen and one electron, no electrons or two electrons respectively. (There are a wide variety of monomers which do not lend themselves to the free-radical type of polymerization. The use of all three mechanism broadens the scope of application of this method). 
represents a unit of vinyl monomer where xe2x80x9cXxe2x80x9d governs the property or properties that are obtained. In many instances a mixture of monomers is employed and often more than one property can be altered in one processing step. These polymer chains whose length can be controlled, are permanently attached to the xe2x80x9csubstratexe2x80x9d. The linkage between the graft-polymer and the substrate is covalent in nature, therefore, the graft-polymer cannot be leached from the substrate. In essence the chemical grafting consisted of growing polymer chains on the backbone chain of a substrate. The graft polymer chains are formed from vinyl monomers or monomer containing appropriate functionability, e.g., groups such as hydroxyl, carboxyl, epoxy, amide, amine anhydride.
Without being bound by any metal grafting theory, the details of which have not been fully established, the mechanism of reaction between the steel surface and monomers or prepolymers is thought to involve a reactive species on the steel surface. In the presence of moisture, there is a layer of oxide and hydroxyl groups tenaciously bound to the steel substrate. The hydrogen of the hydroxyl group may be removed by the graft initiator and form a radical which reacts with the monomer starting graft polymerization. The oxides and hydroxyl groups react with epoxy groups of the prepolymers or monomers starting a chemical reaction which also leads to a strong bonding between the steel and organic polymer formed on the surface.
Thus, the mechanism of graft polymerization may be presented in a series of steps as follows: 
The process of termination may undergo differently when the formulation contain reactive prepolymers or polymers. The prepolymers may undergo also activation by the graft initiator giving reactive radicals P which react with the radical on the steel surface forming a graft coating on the substrate: 
The graft initiator G may consist of the following metal ions; Fe+++/Fe++, Ag+, Co++, Cu++and the peroxide should be chosen from catalysts such as benzoyl peroxide, methyl ethyl ketone peroxide, tert butyl hydroperoxide and hydrogen peroxide. The monomers and prepolymers have side functional groups x, which may react between themselves and with additional prepolymers or polymers included into the formulation forming a graft crosslinked organic coating. The functional groups of the monomers and prepolymers should consist of hydroxyl groups, carbonyl groups, secondary and/or tertiary amino groups and epoxy groups. The molecular ratio of the functional groups of the reactive components should be adjusted so that no free groups should be left after the reaction is finished.
It is accordingly a primary objective of the present invention to provide an improved process for the graft coating of steel surfaces with organic polymer material.
Another object of the invention is to provide organic materials which when coated onto the steel surface effect a graft coating, bonding the coating to the surface.
Yet another object of the invention is to provide steel workpieces, and especially crane parts having a coating of organic polymer material bonded permanently to the steel of the workpiece and adapted to resist corrosion and have desirable wear resistance and frictional characteristics.
A Still further object of the invention is to produce coated steel parts with substantially zero permeability to oxygen and other corrosive gases and with substantially zero water vapor transmission rates.